This invention is directed to an aqueous printing ink composition and a method for improving aqueous printing.
The printing process involves the distribution of ink in a uniform film, the provision of a substrate having a uniform surface, and the transfer of the ink to the substrate in a prescribed pattern. The final transfer of ink from the printing surface to the substrate is accomplished by means of pressure; the final force on the ink film is heavy pressure against a porous surface followed by a tension sufficient to split the ink film.
Flexographic printing is a form of "relief" printing in which an impression is taken from the raised portions of a printing surface. Flexographic printing machines are essentially high-speed web or sheet-fed rotary presses, which print with liquid inks from curved plates, usually made of rubber or other elastomeric or plastic material, attached to a cylinder. An important advantage of flexographic printing is that a uniform film of ink can be printed even on rough papers, because the surface of the rubber plate is sufficiently resilient to be forced into the hollows in the paper.
Flexographic printing can be employed to produce a wide variety of printed material, including waxed or decorative wrappers, aluminum foil, plastic films, newsprint and corrugated papers and cartons. The inking system used in flexographic printing consists essentially of an enclosed duct to limit the evaporation of the solvent, and a simple train of aligned rollers. Ink control is usually obtained by pressure on the end bearings of the inking rollers, which tends to force the rollers apart at the center. Because of this procedure, continuous fine control and correct alignment of rollers is considered to be essential to produce acceptable printing. Although the pressure on the inking rollers may vary appreciably in letter press printing, in contradistinction, flexographic printing requires maintenance of pressure on inking rollers at practically zero. Because of the importance of maintaining such critical inking roller pressure, flexographic inks must be carefully tailored to possess appropriate viscosity or fluidity. In addition, the ink must be sufficiently volatile or penetrative to dry within seconds or less, adhere well to the printing surface, and be odorless when dry. The ink must also be free of any solvents or components which would attack the rubber or plastic printing plates or rollers.
In the case of pigmented inks, which are extensively used because of the color effects and eye appeal of the printed product, the ink should be stable and any pigment which settles out should be readily dispersible by stirring. A considerable amount of research has been expended in attempting to maintain satisfactory flow properties in the inks while at the same time maximizing the degree of pigmentation and improving the ink transfer properties. And at the same time, it is highly desirable to employ water-based inks to avoid organic solvent emissions.
Another widely used printing technique is intaglio printing, which involves taking impressions from recesses engraved or etched below the surface of a plate or cylinder. Presently, the most important type of such printing is rotogravure printing from polished copper electrodeposited on an iron, steel or aluminum base. The design is etched into the copper by photoengraving techniques. Gravure inks rely for drying mainly on solvent evaporation, and therefore, gravure inks usually contain large amounts of hydrocarbon solvents. Not only are these solvents highly flammable, forming explosive mixtures with the atmosphere, but the vapors are also extremely toxic. Because these undesirable properties involve significant expenditures for flameproof electrical equipment, hoods and duct work for containment, and recovery of the solvents, an aqueous rotogravure ink which has suitable ink transfer properties has been needed in the printing field.